Some guidebooks are still wrong, they now charge $10 USD(80 bol) for Extranjeros (strangers). Citizens - locals pay about 10 bol). Getting there is rather easy. If your staying in El Centro (the city), take a taxi to, or a minibus with the "Cemeterio" flap on the window to the Cemetery. Across the street from the cemetery's main gate, there are florists and to the right of this area are minibuses headed to Tiwanaku. You shouldn't pay more than 8-15 bol. Expect a 90 minute drive, buy some snacks and make sure the prices aren't too expensive from them either.
Also, when the drivers head up to El Alto on the road to Tiwanaku, they will most likely pick up more passengers if the bus or minibus isn't full. There usually isn't much traffic on the road out of El Alto, so besides this everything should be fine. some drivers drive fast so they won't waste time, but they are experienced so don't feel nervous. You might also want to choose to sit near a window or out of the sunlight. Also, note that the altitude at Tiwanaku is a bit higher than that of El Alto's, as a tourist your most likely not going to get used to this, so buy some Sorojchi Pills (altitude adjuster pills) from a local pharmacy. You can also book a tour through the many tour operators in La Paz. Most of these appear to be in Spanish only.
The remains of a Pre-Incan civilization, the last skeleton recovered was 13,000 years old, this place is quite magical.
In August of 2007, a large hotel was being built in the town near the ruins. It will probably be completed in another 2 months or so but, there are about 2 motels there that seem fine. There really is no need to stay the night as visiting the ruins should take 4 to 5 hours the most. Perhaps in the future, there will be more to do and see, for now there isn't.
You don't want to leave this area. The area surrounding Tiwanaku and the small town around it is quite barren. You'll see a small mountain range to the West of the ruins and other mountains all around. The main attraction is this site so don't wander around.